In addition to providing initial voice-centric services, a mobile communication system has evolved into a high speed and high quality wireless packet data communication system to provide data and multimedia services. Various mobile communication standards such as HSDPA (high speed downlink packet access), HSUPA (high speed uplink packet access), LTE (long term evolution), and LTE-A (long term evolution advanced) of 3GPP (3rd generation partnership project), HRPD (high rate packet data) of 3GPP2, and 802.16 of IEEE (institute of electrical and electronics engineers) have recently been developed to support high speed and high quality wireless packet data transmission services. In particular, the LTE system, which is a system developed to efficiently support high speed wireless packet data transmission, maximizes wireless system capacity by using various wireless access technologies. The LTE-A system, which is a wireless system evolved from the LTE system, has an enhanced data transmission capability as compared to the LTE system.
The existing 3rd generation wireless packet data communication system such as HSDPA, HSUPA, or HRPD employs technologies for improving transmission efficiency, such as an adaptive modulation and coding (AMC) scheme and a channel response scheduling scheme. When the AMC scheme is employed, a transmitter may adjust the amount of transmitted data according to a channel state. That is, a transmitter may efficiently transmit a large amount of information while adjusting the probability of erroneous reception to a desired level by reducing the amount of transmitted data when a channel state is poor and increasing the amount of transmitted data when a channel state is good. When the channel response scheduling scheme is employed, system capacity is increased as compared to the prior art schemes because a transmitter selectively provides a service to a user having a good channel state among multiple users. Such an increase in system capacity is called multiuser diversity. In short, the AMC scheme and the channel response scheduling scheme allow a transmitter to receive feedback of partial channel state information from a receiver and apply an appropriate modulation and coding technique at a point of time determined to be most efficient.
The AMC scheme may also include a function of determining the number of spatial layers for a transmitted signal, that is, a rank, when being used with a multiple input multiple output (MIMO) transmission scheme. According to this AMC scheme, in determining an optimal data rate, a transmitter considers not only a coding rate and a modulation scheme, but also the number of layers to be used for MIMO transmission.
In recent years, research has been actively conducted to change code division multiple access (CDMA), which is a multiple access scheme used in 2nd and 3rd generation mobile communication systems, to orthogonal frequency division multiple access (OFDMA) in a next generation system. Both 3GPP and 3GPP2 have started standardization work on an evolved system using OFDM. It is known that the OFDMA scheme may be expected to increase capacity when compared to the CDMA scheme. One of many factors increasing capacity in the OFDMA scheme is the capability to perform frequency domain scheduling. As the channel response scheduling scheme makes it possible to obtain capacity gain according to the time-varying characteristic of a channel, more capacity gain may be obtained using the frequency-varying characteristic of a channel.
A mobile communication service using a non-coordinated multi-point (non-CoMP) transmission/reception scheme is conventionally provided by a cellular system including multiple cells implemented as shown in FIG. 1.
FIG. 1 illustrates a conventional mobile communication system including three cells, each of which has a transmission/reception antenna disposed at its center.
Referring to FIG. 1, the conventional mobile communication system includes three cells (cell 0, cell 1, and cell 2) each having cell identification (cell ID)=0, cell ID=1, and cell ID=2. An Enhanced node B (eNB) 1, 10, 20 is disposed in each cell, and thus data transmission/reception may be performed between the eNB 1, 10, 20 and a user equipment (UE) 30, 40, 50 existing in a corresponding cell. That is, the UE0 30 existing in the coverage of the cell 0 receives a physical downlink shared channel (PDSCH) from the eNB 1. At the same as the time when the eNB 1 transmits the data signal to the UE0 30 in the cell 0, the cell 1 and the cell 2 also transmit data signals to the UE1 40 and the UE2 50 by using the same time and frequency resources respectively. The data transmission in the cell 0, the cell 1, or the cell 2 corresponds to the non-CoMP transmission scheme in which radio resources in one cell are used only for a UE within the corresponding cell.
When one UE receives a signal from one cell or transmission point, as shown in FIG. 1, the UE sets a cell or transmission point, from which it is to receive the signal, in advance through radio resource control (RRC) signaling. Also, the UE transmits channel state information (CSI) for a downlink channel, that is, downlink CSI (DL-CSI), only to the set cell or transmission point. The CSI includes information on the data rate supportable by the UE, information on the precoder preferred by the UE, and information on the number of spatial layers, that is, a rank, supportable by the UE in MIMO transmission
Transmission from one transmission point to one UE, as shown in FIG. 1, has a limitation on the receivable data rate when the UE is far away from the transmission point or is subjected to strong interference from neighboring transmission points.